I'm Cydia search for "desktop SMS" I think it's called something like that and then you can go into "settings app" and "wallpaper" and then when you chose and image, it will give you two new options in the form of little grey "i" icons at the Top left and right, one is to set homescreen background and the other, SM background you have to do it in the settings app, the photos app won't do it for some reason. No biggie tho, hope this helps!